Electronic article surveillance (EAS) systems help to deter the unauthorized removal of articles from a surveillance area. One aspect of this deterrence results from the visual observance of a component of an EAS system attached to an article to be protected, namely a resonant tag circuit attached to a particular article. Typical EAS systems involve passing articles having EAS tags attached thereto through an electromagnetic field of predetermined frequency generated in a controlled area. The tag circuit resonates at the frequency of the electromagnetic field, enabling the receiving component of an EAS system to detect the presence of the tag in the controlled area and indicate unauthorized removal of the tagged article from the protected premises. The legitimate removal of tagged articles from the premises may be accomplished by removal or deactivation of the tag circuit before the article is passed through the electromagnetic field.
Typical resonant tag circuits comprise a central dielectric layer and conductive layers on either side of the dielectric layer. For example, U.S. Pat. No. 4,598,276 discloses an EAS marker having a central dielectric layer and conductive spirals on opposite surfaces of the dielectric layer, including an inductive component, the resulting circuit resonating upon exposure to an electromagnetic field of a predetermined frequency.
In addition to the basic component layers of EAS tags described above, various outer or covering layers may be provided over the conductive layers. For example, U.S. Pat. Nos. 4,717,438 and 4,843,404 disclose tags having additional outer layers of an unspecified material and a release sheet attached by means of an adhesive; U.S. Pat. No. 4,792,790 discloses a tag having outer layers of a light-transparent carrier material; and U.S. Pat. No. 4,413,254 discloses a tag having outer cover layers of paper or another insulating material.
Further, U.S. Pat. No. 4,369,557 discloses a tag having outer layers formed of an insulative substrate web having a low dissipation factor at a given frequency and a stable dielectric constant, such as polyethylene, polypropylene, polyester (Mylar), or polyisobutylene. The preferred web material disclosed is polyethylene because of its low cost and ease of bonding with aluminum foil.
Still further examples include U.S. Pat. No. 4,864,280 which discloses a tag having outer layers, one layer being an adhesive film and detachable cover sheet and the opposite layer being a paper film or film of synthetic plastics material. The possibility of printing on the latter layer is also discussed. U.S. Pat. No. 4,783,646 discloses a tag having outer polyester films for use as an insulating covering material which is less expensive than polyimide and facilitates the transmission of radio waves.
The prior art fails to recognize many unexpected advantages arising from the selection of a polymeric material as an outer layer for an EAS tag, such as those advantages associated with a process including indenting or dimpling to fabricate deactivatable tags.
Deactivatable EAS tags are disclosed in U.S. Pat. No. 4,498,076, which is hereby incorporated by reference. This patent discloses deactivatable resonant tag circuits in which a portion of one conductor is indented into the central substrate layer such that the conductors on either side of the substrate are closer to each other at the indented portion than the remainder of the conductors. The tag is deactivated upon exposure to an electromagnetic field of a predetermined frequency, which causes an arc discharge between the conductors through the substrate at the indented portion. The electric arc vaporizes a portion of the conductors near the indented portion, thereby destroying the conductive path and deactivating the circuit. Alternatively, the arc forms a plasma with deposited metal between the conductors which permanently short-circuits the conductors and destroys the resonant properties of the circuit.
Other examples of deactivatable tags are disclosed in the prior art, such as U.S. Pat. No. 4,021,705, which discloses a deactivatable tag having a central layer of electrically insulative material with layers on either side of conductive material and at least one fusible link which can be fused by application of an energizing field to activate or alter the characteristics of the resonant circuit.
Further, U.S. Pat. Nos. 4,778,552, 4,802,944, 4,818,312, 4,846,922, 4,910,499, and 4,954,814 disclose deactivatable tags having an internal non-conductive layer between two conductive layers and outer layers and a release sheet releasably adhered by an adhesive to one of the outer layers.
In some situations, prior art tag circuits may be detuned by contact between the tag and the article to which the tag is secured. For example, meat is typically packaged in a breathable polymeric stretch film which permits liquid or gas, such as oxygen, to pass therethrough. The oxygen which penetrates the film causes the meat packaged therein to have a bright red color associated with freshness. However, the tag circuit may be detuned by capacitance coupling of the inductor with the article. There is a need in the art for a stabilized resonant tag circuit and deactivator in which detuning of the tag circuit by contact with the article or moisture on the article or packaging surface is minimized.